1. Field of the Invention
The invention pertains to a method for predicting radio-wave propagation in the atmosphere. In particular, the invention relates to a hybrid method utilizing a number of different techniques to rapidly predict radio-wave propagation.
2. Description of the Related Art
Radio-waves propagating from a transmitter are controlled by the aggregate effects of the atmosphere's radio refractive index, which may vary in both the vertical and horizontal dimensions. The amplitude of a simple wave front depends upon the accumulated focusing and defocusing caused by the gradient of the refractive index normal to the direction of the wave front propagation at all points along the path of the wave. The phase of such a wave front depends upon the product of the refractive index present at a particular point in the atmosphere times a mathematically determined incremental pathway both of which are integrated over the total path the wave front travels.
The path that a wavefront takes in propagating through a medium is known as a ray. It depends upon the refractive index according to Snell's Law. If a transmitter lies near a reflecting surface, such as the earth, then the transmitted wave will be reflected from this surface. The reflection process modifies the amplitude and phase of the reflected wave based upon the local grazing angle of the wave with the surface, and the refractive indices of the two interfacing mediums. At any point away from a transmitter, the total radio field strength or intensity depends upon the coherent sum of all waves, direct and reflective, present at that point.
Ray Optics methods have long been used to compute radio field strength above a limiting radio wave ray. The limiting radio wave ray is the path of a transmitted radio wave that reflects from a surface at a small limiting angle, generally a fraction of one degree. In the ray optics region lying above the limiting ray, radio field strength is dominated by the interference of rays coming directly from a transmitter and rays reflecting from a reflecting surface. The effects of the refractive index along the direct and reflective wave paths are frequently ignored in this region, but they can be included as described by Hitney et al. in their report referred to above. Ray Optics methods, such as those described in the I. P. Shkarofsky and S. B. Nickerson article referenced above, are known to give good results for waves above the limiting wave ray, but have not been found usable below this limiting ray.
Split-step Parabolic Equation methods have been used to calculate radio field strength for nearly all ranges and altitudes for certain types of transmitters. Descriptions of split-step Parabolic Equation methods are given by G. D. Dockery referred to above. For practical considerations, however, there are limitations as to the maximum height that can be considered by these methods. Parabolic Equation methods depend upon forward and inverse Fast Fourier Transforms to construct range-step marching solutions to the parabolic wave equation, which can readily consider the effects of refractive-index variations in both the horizontal and vertical dimensions.
The vertical spacing between points to be transformed in the Fast Fourier Transforms depends upon frequency, transmitter height above a reflecting surface, and maximum elevation angle. Although no theoretical maximum height exists for the Parabolic Equation method, at microwave frequencies vertical spacing becomes so small that the number of points to be transformed becomes prohibitively large. Because of this, Parabolic Equation methods are typically limited to heights of 1,000 to 2,000 meters.
In FIG. 1 there is shown a computer-generated radio propagation coverage diagram utilizing a commercially available Parabolic Equation method known as PCPEM. PCPEM is available through Signal Science Limited of the United Kingdom. The coverage diagram of FIG. 1 took approximately 82 minutes to construct for a 300-meter surface-based duct.
In FIG. 2 another prior art Parabolic Equation technique has been used to generate a radio field strength coverage diagram for the same 300-meter surface-based duct used in FIG. 1. The program used to generate this diagram is known as RPE and is a government developed research tool written by the Naval Ocean Systems Center of San Diego, Calif. This program took 55 minutes to create the coverage diagram of the 300-meter surface-based duct shown.
In summary, Ray Optics methods provide an effective way of predicting radio-wave propagation effects, but can only be utilized within a small area of the atmosphere above a limiting ray. Parabolic Equation methods are known to be effective at nearly all ranges and altitudes; however, these methods become computationally complex as transmission frequencies increase and altitudes are raised.
For military applications, for example, there is a need for a method that quickly and effectively predicts radio propagation for great ranges and altitudes.